Abstract

In marsupials, both sexes possess a cremaster muscle, that of the female being attached to the mammary gland and surrounding subcutaneous tissues. In the grey short-tailed opossum, the muscle in both sexes has a distinct proximal origin from the anterior superior iliac spine. The number of muscle fibres is higher in male than in female opossums on average, but the difference is not statistically significant. Retrograde tracing, achieved by injecting B-cholera toxin into the muscle, showed that labelled neurons occupied several spinal cord segments, the bulk being in L2–L4. There was no overall sex difference in the number of labelled neurons. It is concluded that cremaster muscles are present in both sexes of the grey short-tailed opossum Monodelphis domestica , contain approximately equal numbers of fibres, and differ only in their insertion target. Since the motor innervation also shows no dimorphisms in this species, it is concluded that the whole neuromuscular complex is homologous in the two sexes. This is in marked contrast to the situation in adult eutherian mammals where the muscle is possessed by males only, and where the cremasteric nucleus is highly dimorphic in terms of motor neuron numbers.